Ahh, you make a great point, that if an instance goes down you lose those keys. Cassandra doesn't necessarily solve this, however, unless you have enough failover that it is unthinkable that you could have cassandra lose data. If you can rebuild the cache in cassandra you can rebuild it in the identical way using my scheme, but my scheme only requires N days of data to be run through.

Step 2 is fine, memcached allows setting of timeouts on keys. If you always set a timeout of (dig_time + five_days) - now() you are set. Be careful, times > 30 days imply a unix timestamp not a timeout.

Mysql kindof sucks. The clustering is easy to set up at least.

Cassandra and other big on disk hash tables are pretty cool. I think once they have datastore-like indexing capabilities they will be totally usable. My qualm with using them is that there are 80 of them right now, and they are all pretty immature. The ones that are mature, like bsddb, and complicated to use.

My point was that cassandra is just a big memcached.